For the first time paleontologists have found fossilized burrows of tetrapods – any land vertebrates with four legs or leglike appendages – in Antarctica dating from the Early Triassic epoch, about 245 million years ago.
The fossils were created when fine sand from an overflowing river poured into the animals' burrows and hardened into casts of the open spaces. The largest preserved piece is about 14 inches long, 6 inches wide and 3 inches deep. No animal remains were found inside the burrow casts, but the hardened sediment in each burrow preserved a track made as the animals entered and exited.
In addition, scratch marks from the animals' initial excavation were apparent in some places, said Christian Sidor, a University of Washington assistant professor of biology and curator of vertebrate paleontology at the Burke Museum of Natural History and Culture at the UW.
"We've got good evidence that these burrows were made by land-dwelling animals rather than crayfish," said Sidor, who is lead author of a paper describing the find, which is being published in the June edition of The Journal of Vertebrate Paleontology.
Co-authors are Molly Miller, a geology professor at Vanderbilt University, and John Isbell, a geosciences professor at the University of Wisconsin-Milwaukee. The work was funded by the National Science Foundation.
Fossils of tetrapod bones from later in the Triassic period have been found in a section of Antarctica called Victoria Land, but the fossil burrows predate those bone fossils by at least 15 million years, Sidor said.
The fossilized burrows were collected in 2003 and 2005-06 from the Fremouw Formation at Wahl Glacier and from the Lashly Formation at Allan Hills, both toward the outer edges of Antarctica.
Despite the absence of fossil bones, the burrows' relatively small size prompted Sidor to speculate that their owners might have been small lizardlike reptiles called Procolophonids or an early mammal relative called Thrinaxodon.
At least they didn't call Thrinaxodon at "mammal-like reptile" this time. The New Scientist all but commits this sin: be careful, the NS draws some conclusions that may - or may not - be true. However, some explanations are in order here.
Traditionally, there are three big groups of amniotes that people talk about, the anapsids, synapsids, and diapsids. These were all based on the number of holes in the skull of an amniote. Anapsids had none. Synapsids have one. Diapsids have two. Of the anapsids, it has been argued, only the turtles remain (and even some contest this!) The rest went extinct between the Permian extinction and some time in the Triassic by and large. The synapsids have also undergone a great winnowing of their numbers. Once there were a great number of different types. Between the Permian Extinction to, possibly, the Paleocene all but one lineage of this amazing group died out. Who is that one remaing lineage? The mammals. You and I and even your cat, though your cat would never admit any kinship. The diapsids actually are probably more diverse than the rest of us: they're the other traditional reptiles (lizards, snakes, etc) and...birds. The two candidate critters according to the article that might have made the burrows are the procolophonids and Thrinaxodon. The Procolophonids were anapsids. Thrinaxodon is a synapsid. Initially, the procs, as I'll call them for now, were largely insectivorous. They filled a role not all that different from modern lizards, but would also become herbivorous not unlike how some lizards do. Thrinaxodon was definitely a carnivore and it definitely known that this, erm, puppy was a burrower.
One of the interesting bits about this is where they were found. If you take a look at where Antarctic was at the time, it was very, very far south. If you take into account the climate, if these burrows were made by procs, then it helps verify that the climate was very warm: the procs were never warmed blooded. Thrinaxodon, otoh, very well may have been warm blooded and, in fact, probably was: the cynodonts, which Thrinaxodon was a member of, were what mammals evolved out of as far as we can tell and based on other information we have from other pre-Thrinaxodon therapsids (a kind of synapsid), endothermy arose during the Permian among this lineage. Other therapsids, for what its worth, that I have written about are the dicynodonts and the gorgonopsids. At some point, hopefully before the end of the year, I am going to write about the therocephalians as well. And the Permian terrestrial ecology. And the No Permian Extinction evolution.
Rockets first though.